1. Field of the Invention
The present invention relates to a cordless telephone which can be connected, with a simple circuit modification, to various digital exchanges of different protocols. Disclosure No. 357466 regarding the present invention has been accepted by the U.S. Patent and Trademark Office.
2. Description of the Prior Art
A digital PBX installed at a node of a business information communication network is provided with a wide range of functions and is frequently referred to as an intelligent PBX (IPBX). However, protocols of such digital PBXs differ from maker to maker. For instance, a digital PBX supplied from a maker employs a line interface for alternately transmitting voice data and control data with two transmission lines for each of voice and control data, while a digital PBX supplied from another maker transmits alternately such voice data and control data with two transmission lines. Moreover, a digital PBX line for ISDN employs four transmission lines in total, two lines for transmission and two lines for reception. In addition, a signal format for signals transmitted to a subscriber telephone set from a digital PBX also differ for each digital PBX. Therefore, if a digital PBX manufactured by a certain maker is installed to constitute a network, cordless telephones to be used in this network must inevitably be those exclusively used for the digital PBX. Therefore, cordless telephone makers have to provide cordless telephones corresponding to digital PBXs of different protocols. The difference in protocol of digital PBXs has prevented the utilization of cordless telephones in offices.
The above explanation can be applied not only to a digital PBX but also to the case where protocols of digital exchanges differ.